This invention relates to methods of forming interference anti-reflective, abrasion resistant and easy to clean coatings on plastic articles by improved microwave plasma modification.
Anti-reflective coating is very popularly used for optical components, infrared sensors and ophthalmic lenses. These optical articles need very low reflectance in special wavelength bands, for instance, 550 nanometers (nm) for ophthalmic lenses and 950 nm for near infrared sensors.
The term "interference anti-reflective coating" used herein refers to a coating with quarter wavelength optical thickness in which the reflectance at minimum point on the spectral reflectance curve shall not exceed 1.6% when the refractive index of the substrate is 1.48-1.50 according to the test prescribed by military specification MIL-C-675C.
Various methods to form the coatings are commercially available. Vacuum evaporation process is a common method of forming single or multilayer interference anti-reflective coatings. This process requires a high level of skill and expensive equipment for manufacturing.
The sputtering process is another vacuum process to form interference anti-reflective coatings. However, this type of equipment for the sputtering process also is very expensive and the process is very difficult to control.
Plasma surface treatment is one significant alternative option. It is a simple and low cost process to form anti-reflective coatings. A method using glow discharge treatment of an organo-silica taught by Masso, U.S. Pat. No. 4,478,873, is known. However, this method can only produce a low reflective coating, reducing the reflectance from 6.8% to 4.2%, and no anti-reflective interference layer according to MIL-C-675C is formed. Masso does not teach how to obtain an interference anti-reflective coating or how to obtain the coating with durable qualities.
U.S. Pat. No. 4,940,602, Taniguchi, teaches a method of forming a fluorine-containing polyorganosiloxy film by the process known as spin coating. '602 limits the index of refraction of the hard coating materials to be not less than 1.52. '602 uses plasma surface treatment to clean the surface to obtain good adhesion between the hard coating and the top anti-reflective coating. Fletcher, in U.S. Pat. No. 4,137,365, uses glow discharge plasma polymerized vinyl- trimethoxysilane to form a low reflective coating. However, the transmittance achieves only 85% at 550 nm. Various treatments have been used for curing, hardening processes as disclosed in U.S. Pat. No. 5,051,308, U.S. Pat. No. 5,156,882, and EP 252,870.
Through normal plasma processes many organic silicon coatings will be ashed, peeled or separated from their substrate and lose the adherence between coating and substrate. To obtain the desired interference property, a stable layer thickness is necessary. This is not only a problem of film thickness but more importantly it is an option of starting material, which will be able to withstand deep structure modification (thicker than 50 nm) without cracking and shall be able to achieve a certain layer thickness (thinner than 500 nm) without further interaction with other layers. If the layer is too thin or too thick, the coating will lose its interference properties.
Yet another object is to obtain a coating with stable layer thickness and a refractive index as low as 1.29-1.35, which is formed from organic silicon by an improved microwave plasma modification. This is one of the main objectives of this invention. This is a basic condition for forming an interference anti-reflective coating.
Another objective is to obtain a high forming rate of the interference anti-reflective layer. This is very important for mass production.
Still another objective is to obtain the interference anti-reflective layer which has good abrasion resistance and good chemical resistance.